Extendable arms are widely used in various kinds of industrial equipment, devices related to daily life, medical-related devices, etc., such as arms of industrial robots, stands of lighting devices, arms supporting a stand on which an article is placed.
A typical extendable arm is formed by a plurality of cross units arranged in one direction and pivotally coupled to each other. Each of the cross units is formed by two linear plate members crossed over each other at their centers so as to form an X-shape and pivotally coupled together at the intersection via a shaft. The extendable arm having such a structure can be extended and contracted in a linear direction.
In some applications, the extendable arm may be extended and contracted in a curved direction. Such extendable arms that are extended and contracted in a curved direction are disclosed in, e.g., Japanese Unexamined Utility Model Application Publication No. H01-159036 (PTL 1) and Japanese Unexamined Patent Application Publication No. 2006-52603 (PTL 2).
The extendable arm disclosed in Japanese Unexamined Utility Model Application Publication No. H01-159036 (PTL 1) is used in a front roll-up type movable tent that is deployed so that canvas forms a curve in an intermediate part of the tent. In the front roll-up type movable tent disclosed in this publication, the canvas is attached to extend between a base bar and a front bar, the front bar is rotatably disposed between the tip ends of extendable arms, and a motor that rotates the front bar in the forward and reverse directions is coupled to the front bar. Each of the extendable arms is formed by a plurality of cross units arranged in one direction and pivotally coupled to each other, and each of the cross units is formed by two linear plate members that are crossed over each other to form an X-shape.
When in a deployed state, the canvas has a curved region in an intermediate part thereof. Each of those cross units which are located in the curved region is provided so that its crossing shaft coupling the two plate members is provided at a position shifted from the centers of the plate members, in order to allow the cross units to be located along such a deployment path of the canvas. The length from the crossing shaft to the outer end of each plate member is greater than that from the crossing shaft to the inner end of each plate member. Accordingly, when the plurality of cross units coupled to each other are deployed, an imaginary line connecting the outer ends of the plate units is curved with a relatively large radius of curvature, and an imaginary line connecting the inner ends of the plate units is curved with a relatively small radius of curvature.
FIG. 12 of Japanese Unexamined Patent Application Publication No. 2006-52603 (PTL 2) shows a telescopic multistage extendable structure having an arch shape that is curved with a predetermined radius of curvature. The extendable structure is formed by placing between a pair of cylindrical arms an extendable mechanism including cross units coupled to each other. Each cylindrical arm is made extendable by telescopically fitting a plurality of cylindrical members having the shape of a bent pipe on each other. Each cylindrical arm thus forms an arch shape when extended. Both ends in the lateral direction of the extendable mechanism including the cross units are pivotally coupled to the cylindrical arms.